1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine.
2. Description of Related Art
Conventionally, a three-way catalytic converter has been known to purify HC, CO, and NOx in exhaust gas of an internal combustion engine. The three-way catalytic converter mainly includes a precious metal, an element, and a oxygen storage medium. The precious metal serves as a catalyst composition, and the element, such as alumina, is used to disperse the precious metal. The oxygen storage medium stores and releases oxygen in exhaust gas.
The three-way catalytic converter functions as an oxygen storage that stores oxygen in exhaust gas by using the oxygen storage medium, and the oxygen storage function is used to improve the exhaust gas purification efficiency. In other words, when the air-fuel ratio of exhaust gas is lean relative to a target air-fuel ratio (or a theoretical air-fuel ratio, for example), the oxygen storage medium of the catalytic converter stores O2. When the air-fuel ratio is rich, O2 stored in the oxygen storage medium is released to exhaust gas in order to oxidize HC and CO.
In an internal combustion engine provided with the three-way catalytic converter, the air-fuel ratio of exhaust gas is periodically switched between rich and lean relative to the target air-fuel ratio at predetermined intervals in order to effectively purify exhaust gas through the above oxygen storage function. Thus, the storage and release of O2 is repeated in the oxygen storage medium of the catalytic converter, and thereby exhaust gas purification performance of the catalytic converter is improved (for example, see JP-A-2005-248884).
Typically, the precious metal serving as the catalyst composition for the three-way catalytic converter includes rhodium (Rh), palladium (Pd), and platinum (Pt). Rh provides the highest NOx purification efficiency. Oxide of Rh is amphoteric oxide, and in contrast, Pd and Pt are basic oxide. Thus, Rh facilitates steam reforming reaction (CmHn+mH2O→(m+n/2)H2+mCO) as compared to the other precious metals, and thereby formation of H2, which is reductant, is facilitated. However, Rh is more expensive as compared to Pt, and thereby there is needed that the precious metal (Pt and Pd) other than Rh is used for the catalyst composition of the three-way catalytic converter. In other words, it is required to develop a catalytic converter without Rh (for example, having Pt instead), which converter still has exhaust gas purification performance equivalent to performance of a catalytic converter having Rh.